This invention relates to clock recovery in a battery powered device, and in particular to a method for recovering a time count in the event that the battery is removed from the device.
It is known from U.S. Pat. No. 6,603,365 to provide a battery backup circuit in a wireless communications device. An oscillator is powered by the battery, and a counter counts pulses generated by the battery. At times when the battery is being replaced, a backup energy storage device, such as a capacitor, provides power to the counter so that the stored time can be saved.
However, this has the disadvantage that no power is supplied to the oscillator while the battery is removed, and so the time during that period is not counted.
U.S. Pat. No. 6,124,764 discloses a device having a second oscillator, which is operable with low power during sleep periods. The low power oscillator is calibrated against the main oscillator of the device during wake-up periods.